1. Field of the Invention
The present invention relates to an engine unit of a vehicle such as an automobile, and specifically to a neighborhood of an intake system thereof.
2. Description of the Related Art
An intake system of an engine unit of a typical motorcycle has an intake tube connected to an intake port of an engine (internal combustion engine), and a throttle valve adjusting an opening degree of the intake tube. In a motorcycle on which a parallel multi-cylinder engine is mounted, a space in which a throttle body or an intake tube is disposed is generally smaller compared with that of a four-wheel vehicle. Thus, in Patent Documents 1, 2, constitutions in each of which a drive motor driving a throttle valve is disposed on a same side as that of a fuel injection valve are proposed. In these constitutions described in Patent Documents 1, 2, it is possible to dispose a drive motor in a vacant space generated in a case where a fuel injection valve is disposed in an intake tube, so that an increase in a size of the engine unit can be avoided.
Further, since the space in which the intake tube is provided is small, there is used a constitution in which the intake tube is extended to the inside of an air cleaner in order to obtain a necessary intake tube length. However, such a constitution causes a problem that a capacity of the air cleaner is decreased. In particular, in an engine where great importance is put on torque improvement in a low to medium rotation area, an intake tube length is tend to be set longer. In such an engine, the capacity of the air cleaner is decreased further, and output reduction (in particular, transient characteristic reduction) is brought about.
Besides, in an intake system of an engine unit of a motorcycle, in order to prevent output reduction due to intake interference, an independent throttle body in which a throttle valve is provided in each cylinder is adopted. Further, in order to improve an engine response by a throttle operation, a constitution in which a throttle valve is placed to a cylinder of an engine unit as close as possible is generally used.
However, in such an intake system, an intake inertia effect to be obtained becomes small in a slightly open region of the throttle valve. This is because in the slightly open region of the throttle valve, an intake tube length for obtaining the intake inertia effect is not a full length of an actual intake tube but is a length between an intake valve and the throttle valve. As a result, a charge efficiency of the slightly open region being a regular region becomes low. Then, a driver drives increasing a rotation number of an engine in order to obtain a necessary torque, bringing about aggravation of fuel consumption.
Further, the independent throttle body adopted in the motorcycle is manufactured by aluminum die casting. Such a constitution makes the independent throttle body heavier and more expensive, compared with a resin intake manifold for a four-wheel vehicle or the like.
[Patent Document 1] Japanese Laid-open Patent Publication No. 2002-256895
[Patent Document 2] Japanese Laid-open Patent Publication No. 2002-256896
In a conventional engine, in an intake system in particular, an intake tube is extended to the inside of an air cleaner, so that a substantial air cleaner capacity which can be used in an intake process of the engine is decreased by an amount of an inner capacity of the intake tube. An intake tube length is tend to be set longer in an engine aiming at torque improvement in a low and medium speed region in particular, and the air cleaner capacity is further decreased in such an engine, which results in output reduction (transient characteristic, in particular).
Further, in a layout of an independent throttle body type, an inertia effect obtained in a throttle valve subtly open region is small. This is because in the throttle valve subtly open region an actual intake tube full length is not an intake tube full length itself but a length from an intake valve to a throttle valve in a throttle body. As a result, a charging efficiency in the throttle valve subtly open region being a standard region is low, leaving no choice but to drive while increasing a rotation number in order to obtain a necessary torque, so that fuel consumption becomes worse if no measure is taken.